I suppose I'll take a shot at a counterargument.

The problem with the military using an off the shelf system like Signal isn't the tangible features (re: encryption, usable ux). The problem is one of control. What happens when the government gets into a dispute with Signal (say, over a warrantless wiretap) and Signal decides in protest that it will no longer have any DoD affiliated customers? What happens when, through a series of shell companies and legal loopholes, a controlling stake in Signal ends up in Chinese or Russian hands?

From the PoV of the DoD, owning the system from end to end is a feature they shouldn't compromise on, even if the other features suffer and it costs more than it otherwise would.